Northumberland Strait CrossingConfederation Bridge

New Brunswick, Canada

Will AstleyJonathan Richards

Outline

• Introduction• Geology• Foundation Design Considerations• Solution• Construction Sequence

Introduction

• Longest bridge in Canada at 13 km– Longest bridge in the world spanning ice-covered water

• Crucial transport link between PEI and New Brunswick– Replaced ferry service over the strait

• Completed in May 1998 after 4 years of construction• Total construction cost $1 bn CAN

Geology - History• Developed in pre-glacial times• Permo-carboniferous rock overlain by glacial and marine

sediments• Surfical deposits ranging from fine sands and silts to 1 m

diameter sandstone boulders• Glacial till the predominant deposit varying thickness of 0.5

– 12 m• In post-glacial times there was a number of changes in sea

level• Between 13,000 – 7,000 B.P. Glacial rebound and sea level

changes produced undulating land• Estimated at around 5,000 B.P. the strait became a

continuous body of water

Geology - Stratigraphy

• Extreme vertical/horizontal variations owing to the estuarine/fluvial environment of the strait

• Complex interlayered sequence of sandstones, mudstones, siltstones and conglomerate

Geology – Rock Types

• Glacial Till: varied grading, mixture of sand/silt, 193 kPa, φ’ = 34°, c’ = 95 kPa

• Sandstone: fine/medium grained, ranging from well to poorly cemented, 16 – 22 Mpa, oriented with land

• Mudstone: poor quality, indurated rock/hard clay, 3.5 Mpa, oriented with bedding planes

• Soft Mudstone: “silty clay”, friable/brittle, 43 – 280 kPa, φ’ = 15°, oriented with bedding planes

Foundation Design Considerations

• Loading – Ice, Wind, Impact• Short Construction Window• Overburden (up to 12m in places)• Soft Mudstone Issues – Bearing Capacity / Sliding

Resistance• Seabed up to 40m below water surface• Three main criteria– Bearing Resistance– Sliding Resistance– Settlement

Foundation Design Considerations

• Bearing Resistance– Rock Mass Rating (RMR) classification system used

for jointed rock mass.• Use Engineering experience, judgement, geological

review, core analyses and case record studies

• Sliding Resistance– Need to resist lateral forces

• Settlement– Computer Analyses Conducted – 10mm predicted

Construction Sequence

• Dredge Overlying Material• Excavate Trench• Grout Bags• Place Pier• Tremie Concrete